Conventionally, hydraulic-type and electric-motor-type drive apparatus have been used for generating thrust force in various mechanical apparatus.
In a molding machine, which is one type of mechanical apparatus; for example, an injection molding machine, a resin is melted within a heating cylinder through application of heat; the thus-molten resin is injected under high pressure into a cavity of a mold apparatus so as to fill the cavity; and the resin within the cavity is cooled to set, thereby yielding a molded article.
The injection molding machine has a mold apparatus, a mold clamping apparatus, and an injection apparatus. The mold apparatus includes a stationary mold and a movable mold. The mold clamping apparatus includes a stationary platen to which the stationary mold is attached, a movable platen to which the movable mold is attached, and a mold clamping mechanism. The mold clamping mechanism is activated so as to cause the movable platen and the movable mold to advance and retreat, whereby the mold apparatus can perform mold closing, mold clamping, and mold opening. The injection apparatus includes a heating cylinder and a screw, which is disposed within the heating cylinder rotatably and in a manner capable of advancing and retreating. By means of causing the screw to advance, the resin is injected into the cavity of the mold apparatus from an injection nozzle attached to an end of the heating cylinder so as to fill the cavity.
In some cases, the injection apparatus uses a hydraulic-type or electric-motor-type drive apparatus for rotating and causing the screw to advance and retreat.
In the hydraulic-type drive apparatus, an injection cylinder is disposed behind a screw, and hydraulic pressure is applied to the hydraulic chamber of the injection cylinder so as to cause a piston connected to the screw to advance and retreat, whereby the screw can be caused to advance and retreat. Also, a hydraulic motor is disposed behind the injection cylinder. An output shaft of the hydraulic motor is connected to the piston. By means of driving the hydraulic motor, the screw can be rotated.
In the electric-motor-type drive apparatus, a pressure plate, which supports a screw such that the screw can rotate, is disposed in a manner capable of advancing and retreating in relation to a front plate that supports a heating cylinder; and, by means of driving an electric metering motor attached to the pressure plate, the screw can be rotated. A ball screw is disposed between the pressure plate and an electric injection motor. By means of driving the injection motor and converting rotary motion to rectilinear motion by means of the ball screw, the screw can be caused to advance and retreat.
In the hydraulic-type drive apparatus, the hydraulic motor for rotating the screw is disposed on a rotational shaft of the screw, so that the moment of inertia can be reduced. By contrast, in the electric-motor-type drive apparatus, the metering motor for rotating the screw cannot be disposed on the rotational axis of the screw, so that the moment of inertia cannot be reduced. Accordingly, the electric-motor-type drive apparatus fails to exhibit high-speed performance, high responsiveness, and high control accuracy.
In order to cope with the above problems, an injection apparatus that uses a linear motor as a drive apparatus can be conceived. In the injection apparatus, the linear motor includes a movable element having a permanent magnet, and a stationary element having coils. Supply of predetermined current to the stationary element causes the movable element to advance and retreat, thereby causing the screw to advance and retreat. In this case, magnetic poles of the permanent magnet are arranged such that N poles and S poles alternate, and the coils are wound in a manner corresponding to the magnetic poles. Accordingly, the drive apparatus can exhibit high-speed performance, high responsiveness, and high control accuracy.
In some cases, the mold clamping apparatus also uses a hydraulic-type or electric-motor-type drive apparatus.
In the hydraulic-type drive apparatus, a mold-clamping cylinder is disposed behind a movable platen, and hydraulic pressure is applied to the hydraulic chamber of the mold-clamping cylinder, thereby causing the movable platen to advance and retreat.
In the electric-motor-type drive apparatus, an electric mold-clamping motor, which is attached to a toggle support disposed behind a movable platen, is activated, and a ball screw converts rotary motion to rectilinear motion so as to cause a crosshead to advance and retreat, thereby causing the movable platen, via a toggle mechanism, to advance and retreat.
However, in the case of the hydraulic-type drive apparatus, in order to apply hydraulic pressure to the hydraulic chamber of the mold-clamping cylinder, a hydraulic circuit must be disposed for supplying the hydraulic chamber oil discharged from a hydraulic pump. In the case of the electric-motor-type drive apparatus, after rotary motion generated through drive of the mold-clamping motor is converted to rectilinear motion, a mold-clamping force must be generated by means of the toggle mechanism. The drive apparatus of either type fails to exhibit high-speed performance, high responsiveness, and high control accuracy.
In order to cope with the above problems, use of a linear motor as a drive apparatus is conceived. In the mold-clamping apparatus, the linear motor includes a movable element having a permanent magnet, and a stationary element having coils. Supply of predetermined current to the stationary element causes the movable element to advance and retreat, thereby causing the crosshead to advance and retreat and thus causing the movable platen to advance and retreat. Also, in this case, magnetic poles of the permanent magnet are arranged such that N poles and S poles alternate, and the coils are wound in a manner corresponding to the magnetic poles. Accordingly, the drive apparatus can exhibit high-speed performance, high responsiveness, and high control accuracy.
However, since the linear motor does not have a decelerating mechanism, use of the linear motor as the conventional drive apparatus; i.e., use of the linear motor as a drive apparatus for an injection molding machine, which is a high-load mechanical apparatus, requires an increase in the capacity of the linear motor in order to generate a large thrust force and to provide short-cycled continuous drive.
The capacity of the linear motor can be increased by increasing the area of the permanent magnet used in the linear motor. However, since the linear motor generally assumes a flat shape, increasing the area of the permanent magnet increases the size of the linear motor, resulting in an increase in not only the size of the drive apparatus, but also the cost of the drive apparatus. Also, when the area of the permanent magnet is increased, the weight of the movable element increases accordingly, resulting in a failure for the drive apparatus to exhibit high-speed performance and high responsiveness.
In the case where the linear motor is used in the injection apparatus, since the linear motor for effecting injection and the metering motor are disposed adjacent to each other on the same straight line, the axial dimension of the injection apparatus increases, resulting in an increase in the size of the injection apparatus.
An object of the present invention is to solve the above-mentioned problems in the conventional drive apparatus and to provide a drive apparatus for an injection molding machine which can generate a large thrust force, can provide short-cycled continuous drive, and can be reduced in size and cost, as well as an injection apparatus and a mold-clamping apparatus.